1. Field of the Invention
This invention relates to automatic transmissions having a layshaft kinematic arrangement, particularly to such transmissions having input clutches but no torque converter.
2. Description of the Prior Art
Automatic transmissions for transmitting power between an input and an output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open hydraulic friction clutches and brakes, hydraulic pump, and gear meshes.
To improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce all of these parasitic losses except gear mesh losses. An ASM transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next speed ratio, and then restoring torque. A primary functional feature of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change because there is no torque converter or producing a hydrokinetic connection between the engine and transmission input.
Dual clutch layshaft transmissions are essentially two ASM transmissions, one providing odd numbered gears and one providing even numbered gears. Shifts between odd numbered gears and even numbered gears can be accomplished without interrupting power flow. While operating in an odd numbered gear, the couplers can be moved to configure the transmission for the next even numbered gear. Dual clutch transmissions have parasitic losses only slightly higher than ASM transmissions.
Layshaft ASM transmissions offer significant efficiency improvements over traditional automatic transmissions with torque converters. However, ASMs must produce more torque multiplication in the lower gears than would be required of a transmission having a torque converter in order to compensate for the torque multiplication that a torque converter produces at lower speeds. Layshaft ASM transmissions must produce more torque multiplication in the lower gears also to avoid excessive energy into the clutch during launch of a vehicle from a stop condition.
A transmission having a large span usually requires many gear ratios to keep the ratio steps small. Consequently, there are many gears and synchronizer or couplers in large span transmissions.